In radiography, sometimes an image of a region with long length is taken, such as the entire spine or the legs. In Computed Radiography (CR), images are taken with Imaging Plates (IP) which partially overlap each other and a long length image is created by combining the partial images. Accurate alignment and measurement can be obtained by superimposing an object of known geometry covering the region to be imaged and correcting and aligning the partial images to reconstruct the object of known geometry (see EP0919858, EP0866342). This technique does not suffer from patient movement since all images are acquired in a single X-ray exposure.
In recent years, Digital Radiography (DR) has become a valuable alternative for CR. The flat panel detectors (FPD) used in DR are more costly than the IPs for CR, so an alternative to the one-shot long length imaging technique of CR is needed. This is achieved by taking plural partial images by moving the position of the FPD while turning the X-ray tube or moving the X-ray tube parallel to the FPD and pasting the partial images to obtain a composed long length image. During this FDP and X-ray tube movement, the patient may move, hereby introducing artifacts which need to be compensated in the image composition.
Long length images are mostly taken to perform length and angle measurements on the subject across an area larger than a single FPD.
It is therefore important to create an image where the alignment of the partial images of the subject and the calibration is accurate. If the patient is capable to stand still perfectly, this can be accomplished with known techniques such as described in the co-pending European application EP11157111.3 filed on Mar. 7, 2011 entitled ‘Radiographic imaging method and apparatus’. Unfortunately, most patients requiring a long length examination suffer from a condition which makes it difficult to stand still during the time of the image acquisition. Therefore, the partial images are positioned with a certain amount of overlap such that a user is capable of positioning the partial images to form a composite image based on the image content in the area of the overlap.
In order to reduce the amount of X-ray exposure, images with different sizes are needed. This can be accomplished by manipulation of the collimator which is adjusted to block the x-rays based on the size of the exposure, hereby reducing exposure to the subject.
European patent application 1 484 016 A1 relates to the acquisition of a composite image with a digital detector. Positions of individual component images of the composite image are calculated. First the structures that tend to move are identified and then non-uniform collimation angles may be used for the different component images to avoid placing these structures in the regions of overlap. This method requires identification or estimation of the position of these structures.
It is an object of the present invention to optimize the calculation of the sizes of the partial images so as to reduce the above-mentioned problems occurring in imaging elongate bodies and originating from patient movement during multiple shot exposure and image recording.